Bhikkhus, whatever kinds of worldly merit there are, all are not worth one sixteenth part of the heart-deliverance of loving-kindness; in shining and beaming and radiance the heart-deliverance of loving-kindness far excels them.

Just as whatever light there is of stars, all is not worth one sixteenth part of the moon’s; in shining and beaming and radiance the moon’s light far excels it; and just as in the last month of the rains, in the autumn when the heavens are clear, the sun as it climbs the heavens drives all darkness from the sky with its shining and beaming and radiance; and just as, when night is turning to dawn, the morning star is shining and beaming and radiating; so too, whatever kinds of worldly merit there are, all are not worth one sixteenth part of the heart-deliverance of loving-kindness; in shining and beaming and radiance the heart-deliverance of loving-kindness far excels them.~ Buddha (Itivuttaka sutta 27)

And how does a bhikkhu abide with his heart imbued with loving kindness extending over one direction? Just as he would feel friendliness on seeing a dearly beloved person, so he extends loving-kindness to all creatures.~ The Buddha (Abhidhamma pitaka - Appamannavibhanga)

Metta sutta

What should be done by one skillful in good
So as to gain the State of Peace is this:
Let him be able, and upright and straight,
Easy to speak to, gentle, and not proud,
Contented too, supported easily,
With few tasks, and living lightly;
His faculties serene, prudent, and modest,
Unswayed by the emotions of the clans;
And let him never do the slightest thing
That other wise men might hold blamable.
(And let him think:) “In safety and in bliss
May creatures all be of a blissful heart!
Whatever breathing beings there may be
No matter whether they are frail or firm,
With none expected, be they long or big
Or middle-sized, or be they short or small
Or thick, as well as those seen or unseen,
Or whether they are dwelling far or near,
Existing or yet seeking to exist.
May creatures all be of a blissful heart!
Let no one work another one’s undoing
Or even slight him at all anywhere:
And never let them wish each other ill
Through provocation or resentful thought.”
And just as might a mother with her life
Protect the son that was her only child,
So let him then for every living thing
Maintain unbounded consciousness in being;
And let him too with love for all the world
Maintain unbounded consciousness in being
Above, below, and all round in between,
Untroubled with no enemy or foe.
And while he stands or walks or while he sits
Or while he lies down, free from drowsiness,
Let him resolve upon this mindfulness:
This is Divine Abiding here, they say.
But when he has no trafficking with views,
Is virtuous, and has perfected seeing,
And purges greed for sensual desires,
He surely comes no more to any womb.~ The Buddha (Metta Sutta from Sutta Nipata)

Benefits of loving kindness

Bhikkhus, when the heart-deliverance of loving-kindness is maintained in being, made much of, used as on’s vehicle, used as one’s foundation, established, consolidated, and properly managed, then eleven blessings can be expected. What are the eleven?

One sleeps in comfort; one wakes in comfort; one dreams no evil dreams; one is dear to human beings; one is dear to non-human beings; the gods guard one; no fire or poison or weapon harms one; one’s mind can be quickly concentrated; the expression of one’s face is serene; one dies without falling into confusion; and, even if one fails to penetrate any further, one will pass on to the world of High Divinity, to the Brahma world.~ The Buddha (Anguttara Nikaya 11:16)

Methodical practice of Loving Kindness: from the Patisambhidamagga – traditionally ascribed to the Arahant Sariputta (A Chief Disciple of the Buddha)

The heart-deliverance of loving kindness is practiced with unspecified extension, with specified extension, and with directional extension.

That with unspecified extension is practiced in five ways as follows: May all beings be freed from enmity, distress, and anxiety, and may they guide themselves to bliss.

May all breathing things…all creatures…all persons…May all those who are embodied be free from enmity, distress, and anxiety, and may they guide themselves to bliss.

That with specified extension is practiced in seven days as follows: May all women be freed from enmity, distress, and anxiety, and may they guide themselves to bliss. May all men…all noble ones…all who are not noble ones…all deities…all human beings…May all those in the states of deprivation be feed from enmity, distress, and anxiety, and may they guide themselves to bliss.

That with directional extension is practiced in ten ways as follow:

May all beings in the eastern direction be freed from enmity, distress, and anxiety, and may they guide themselves to bliss. May all beings in the western direction in the northern direction… in the northern direction… in the southern direction…in the eastern intermediate direction…in the western intermediate direction… in the northern intermediate direction… in the southern intermediate direction… in the downward direction…May all those in the upward direction be freed from enmity, distress, and anxiety, and may they guide themselves to bliss.

May all breathing things…May all creatures…May all persons…May all who are embodied…May all women…May all men…May all noble ones…May all who are not noble ones…May all deities…May all human beings…

May all those in the states of deprivation in the eastern direction be freed fro enmity, distress, and anxiety, and may they guide themselves to bliss…May all those in states of deprivation in the upward direction be freed from enmity, distress, and anxiety, and may they guide themselves to bliss.~ Venerable Sariputra (A Chief Disciple of the Buddha)

The Buddha:

Bhikkhus, there are these five ways of removing annoyance, by which annoyance can be entirely removed by a bhikkhu when it arises in him. What are the five?

Loving-kindness can be maintained in being towards a person with whom you are annoyed: this is how annoyance with him can be removed. Compassion can be maintained in being towards a person with whom you are annoyed; this too is how annoyance with him can be removed. Equanimity can be maintained in being towards a person with whom you are annoyed; this too is how annoyance with him can be removed. The forgetting and ignoring of a person with whom you are annoyed can be practiced; this too is how annoyance with him can be removed. Ownership of deeds in a person with whom you are annoyed can be concentrated upon thus: “This good person is owner of his deeds, heir to his deeds, his deeds are the womb from which he is born, his deeds are his kin for whom he is responsible, his deeds are his refuge, he is heir to his deeds, be they good or bad.” This too is how annoyance with him can be removed. These are the five ways of removing annoyance, by which annoyance can be entirely removed in a bhikkhu when it arises in him.~ The Buddha (Anuguttara Nikaya, 5:161)

Bhikkhus, there are five modes of speech that others may use when they address you. Their speech may be timely or untimely, true or untrue, gentle or harsh, for good or harm, and may be accompanied by thoughts of loving kindness or by inner hate.

Suppose a man came with a hoe and a basket, and he said, “I shall make this great earth to be without earth”; and he dug here and there and strewed here and there, and sat here and there, and relieved himself here and there, saying, “Be without earth, be without earth.” What do you think, bhikkhus, would that man make this great earth to be without earth? _ No, venerable sir. Why is that? Because this great earth is deep and measureless; it cannot possibly be made to be without earth. So the man would reap only weariness and disappointment.

Suppose a man came with lack or gamboges or indigo or acarmine, and he said, “I shall draw pictures, I shall make pictures appear on this empty space. “What do you think bhikkhus, would that man draw pictures, would he make pictures appear, on that empty space? – No, venerable sir. Why is that? Because that empty space is formless and invisible; he cannot possibly draw pictures, make pictures appear there. So the man would reap weariness and disappointment.

So too bhikkhus, there are these five modes of speech that others may use when they address you. Their speech may be timely or untimely, true or untrue, gentle or harsh, for good or for harm, and maybe accompanied by thoughts of loving kindness or by inner hate. Now this is how you should train yourselves here: “Our minds will remain unaffected, we shall utter no bad words, we shall abide friendly and compassionate, with thoughts of loving –kindness in and no inner hate. We shall abide with loving-kindness in our hearts extending to that person, and we shall dwell extending it to the entire world as our object, with our hearts abundant, exalted, measureless in loving-kindness, without hostility or ill-will.” That is how you should train yourselves.

Even were bandits savagely to sever you limb from limb with a two-handled saw, he who on that account entertained hate in his heart would not be one who carried out my teaching. Bhikkhus you should keep this instruction on the Similie of the Saw constantly in mind.~ The Buddha (MN 21)

Bhikkhus, seven things gratifying and helpful to an enemy befall one who is angry, whether a woman or a man. What are the seven?

Here, bhikkhus, an enemy wishes thus for his enemy: “Let him be ugly.” Why is that? No enemy relishes an enemy’s beauty. Now when this person is angry, a prey to anger, ruled by anger, be he ever so well bathes\d and well anointed, with hair and beard trimmed, and clothed in white, yet he is ugly through his being a prey to anger. This is the first thing gratifying and helpful to an enemy that befalls one who is angry, whether a woman or man.

Also an enemy wishes thus for his enemy: “Let him lie in pain.” Why is that? No enemy relishes an enemy’s lying in comfort. Now when this person is angry, a prey to anger, ruled by anger, for all he may lie on a couch spread with rugs, blankets, and counterpanes with a deerskin cover, a canopy, and red cushions for the head and feet, yet he lies only in pain through his being a prey to anger. This is the second thing gratifying to an enemy that befalls one who is angry, whether a woman or a man.

Also an enemy wishes thus for his enemy: “Let him have no prosperity.” Why is that? No enemy relishes an enemy’s prosperity. Now when this person is angry, a prey to anger, he mistakes bad for good and he mistakes good for bad, and each being taken wrongly in the other’s sense, these things for long conduce to his harm and suffering, through his being a prey to anger. This is the third thing gratifying and helpful to an enemy that befalls one who is angry, whether a woman or a man,

Also an enemy wishes thus for his enemy: “Let him not be rich.” Why is that” No enemy relishes an enemy’s having riches. Now when a person is angry, a prey to anger, should he have riches gained by endeavour, built up by the strength of his arm, earned by sweat, lawful and lawfully acquired, yet the king’s treasury gathers (in fines) through his being a prey to anger. This is the fourth thing gratifying and helpful to an enemy that befalls one who is a prey to anger, whether a woman or a man.

Also an enemy wishes thus for his enemy: “Let him not be famous.” Why is that? No enemy relishes an enemy’s having fame. Now when a person is angry, a prey to anger, ruled by anger, what fame he may have acquired by diligence he loses through his being a prey to anger. This is the fifth thing gratifying and helpful to an enemy that befalls one who is a prey to anger, whether a woman or a man.

Also an enemy wishes thru for his enemy: “Let him have no friends, “ Why is that? No enemy relishes an enemy’s having friends. Now when this person is angry, a prey to anger, ruled by anger, the friends he may have, his companions, relatives, and kin, will deep away from his through his being a prey to anger. This is the sixth thing gratifying and helpful to an enemy that befalls one who is a prey to anger, whether a woman or a man.

Also an enemy wishes thus for his enemy: “Let him, on the dissolution of the body, after death, reappear in a state of deprivation, in a bad destination, in perdition, even in hell.” Why is that? No enemy relishes an enemy’s going to a good destination. Now when this person is angry, a prey to anger, ruled by anger, he misconducts himself in speech, and mind, on the dissolution of the body, after death, he reappears in a state of deprivation, in a bad destination, in perdition, even in hell, through his being a prey to anger. This is the seventh thing gratifying and helpful to an enemy that befalls one who is angry, whether a woman or a man.

When anger does possess a man,
He looks ugly; he lies in pain;
What benefit he may come by
He misconstrues as a mischance;
He loses property (through fines)
Because he has been working harm
Through acts of body and speech
By angry passion overwhelmed;
The wrath and rage that madden him
Gain him a name of ill-repute;
His fellows, relatives and kin,
Will seek to shun him from afar;
And anger fathers misery:
This fury does so cloud the mind
Of man that he cannot discern
This fearful inner danger.
An angry man no meaning know,
No angry man sees the Dhamma,
So wrapped in darkness, as if blind,
Is he whom anger dogs.

Someone a man in anger hurts;
But, when his anger is later spent
With difficulty or with ease,
He suffers as if seared by fire.
His look betrays the sulkiness
Of some dim smoky smoldering glow.
Whence may flare up an anger-blaze
That sets the world of men aflame.
He has no shame or conscience curb,
No kindly works come forth from him,
There is no island refuge for
The man whom anger dogs.

Such acts as will ensure remorse,
Such as are far from the true Dhamma:
It is of these that I would tell,
So harken to my words.

Anger makes man a parricide,
Anger makes him a matricide,
Anger can make him slay the saint
As he would kill the common man.
Nursed and reared by a mother’s care,
He comes to look upon the world,
Yet the common man in anger kills
The being who gave him life.

No being but seeks his own self’s good,
None dearer to him than himself,
Yet men in anger kill themselves,
Distraught for reasons manifold:
For crazed they stab themselves with daggers,
In desperation swallow poison,
Perish hanged by ropes, or fling
Themselves over a precipice.
Yet how their life-destroying acts
Bring death unto themselves as well,
That they cannot discern, and that
Is the ruin anger breeds.

This secret place, with anger’s aid,
Is where mortality sets the snare.
To blot it out with discipline,
With vision, strength, and understanding,
To blot each fault out one by one,
The wise man should apply himself,
Training likewise in the true Dhamma;
“Let smoldering be far from us.”
Then rid of wrath and free from anger,
And rid of lust and free from envy,
Tamed, and with anger left behind,
Taintless, they reach Nibbana.~ The Buddha (Anuguttara Nikaya 7:60)